The present invention relates to a superconducting wire and, more particularly, to an aluminum-stabilized Nb.sub.3 Sn fine multifilamentary superconducting wire which exhibits a superior stability.
The Nb.sub.3 Sn fine multifilamentary superconducting wire has been used as the wires for a superconducting magnet which can generate a strong magnetic field of 10 tesla or higher. The most conventional example of the Nb.sub.3 Sn fine multifilamentary superconducting wire is shown in FIGS. 1a and 1b. This superconducting wire 1 has a copper member 6 in which are embedded a plurality of bundles 5 each consisting of a multiplicity of Nb.sub.3 Sn fine filaments 3 embedded in a Cu-Sn bronze 2 which in turn is enveloped by a diffusion barrier layer 4. FIG. 1a shows an example of the Nb.sub.3 Sn fine multifilamentary superconducting wire 1 having a circular cross-section, while FIG. 1b shows an example of that having, a rectangular cross-section.
Generally, from the viewpoint of engineering, the Nb.sub.3 Sn fine filament 3 has a diameter of about several micron meters (.mu.m) and the number of the Nb.sub.3 Sn fine filaments contained in one bundle 5 ranges from several tens to several thousands. Further, the number of the bundles 5 contained in one Nb.sub.3 Sn fine multifilamentary superconducting wire 1 ranges from several to several hundreds. In order to obtain a high electromagnetic stability, the superconducting wire usually contains a metallic material having a small specific electric resistance as a stabilizing material. In the superconducting wires shown in FIGS. 1a and 1b, the Cu member 6 serves as the stabilizing material. In general, an oxygen-free copper is used as the material of copper member 6.
In a process for producing the Nb.sub.3 Sn fine multifilamentary superconducting wire 1, there is required a diffusion heat treatment for forming Nb.sub.3 Sn. During the diffusion heat treatment, the Sn in the Cu-Sn bronze 2 tends to diffuse into the copper member 6 thereby contaminating the copper. This, however, is prevented by the diffusion barrier layer 4.
In the production of a superconducting magnet, a factor .alpha. given by the following formula (1) is used as a stabilizing parameter: ##EQU1## where, .rho..sub.st : specific electric resistance of stabilizing material,
Id: electric current in superconducting magnet, PA1 A.sub.st : cross-sectional area of stabilizing material, PA1 p: effective circumferential length of cooling surface, and PA1 q: heat flux from cooling surface.
The smaller the value of the factor .alpha. obtained from the formula (1) is, the higher the stability becomes. In particular, when the value of the factor .alpha. is smaller than 1, a state called "perfectly stabilized state" is attained, in which the cooling power can overcome the Joule's heat even when the superconducting state is broken.
Now, a smaller value of the factor .alpha. can be obtained by using a material having a small specific electric resistance .rho..sub.st appearing in the formula (1), provided that the geometrical size is constant. Since the specific electric resistance at cryogenic temperatures of a high-purity aluminum (purity higher than 99.99%) is as small as 1/5 to 1/10 of that of the oxygen-free copper, when the high-purity aluminum is used as a stabilizing material instead of the oxygen-free copper, the electromagnetic stability is extremely improved. However, since the high-purity aluminum is very ductile, it brings about a problem in that, when a composite body as the fine multifilamentary superconducting wire is subjected to a working such as drawing, there is too large difference in plastic workability between it and the other components. In consequence, this problem has greaty restricted the realization of the fine multifilamentary superconducting wire stabilized by aluminum.
FIGS. 2a and 2b show hitherto known examples of aluminum-stabilized fine multifilamentary superconducting wire. More specifically, FIG. 2a shows a cross-section of an aluminum-stabilized alloy system multifilamentary superconducting wire 9 in which a plurality of filaments 8 of a superconducting alloy, e.g., Nb-Ti, Nb-Ti-Zr or the like, are embedded in the high-purity aluminum member 7. On the other hand, FIG. 2b shows a cross-section of an aluminum-stabilized alloy system fine multifilamentary superconducting wire 9 having a construction in which a plurality of alloy system fine multifilamentary wires 10 and a plurality of high-purity aluminum wires 11 are stranded together and united by a solder 12 such as of Pb-Sn. Each of the alloy system fine multifilamentary superconducting wire 10 has a multiplicity of superconducting alloy filaments 8 embedded in the copper member 6.
As shown in FIGS. 2a and 2b, all of the known aluminum-stabilized fine multifilamentary superconducting wires apply to the alloy system superconductor and are not the one in which the Nb.sub.3 Sn superconductor is stabilized by aluminum. The reasons why such superconducting wire, i.e., a superconducting wire in which the Nb.sub.3 Sn superconductor is stabilized by aluminum, has not been obtained hitherto are attributable to the following facts. Namely, for example, if it were attempted to apply the construction shown in FIG. 2a to the Nb.sub.3 Sn fine multifilamentary superconducting wire, in other words if it were attempted to replace the copper member 6 in FIG. 1a or 1b with an aluminum member, the following problems would arise: firstly, since there is a large difference in plastic workability between the aluminum member and the other components, it is extremely difficult to execute sufficient drawing work of the composite body to obtain the intended fine multifilaments of Nb.sub.3 Sn and, secondly, when the drawn composite body is subjected to a diffusion heat treatment at a temperature range of between 600.degree. and 800.degree. C. to form the Nb.sub.3 Sn superconductor, the aluminum member is melted or softened thereby making it impossible to maintain the shape of the composite body. Further, for example, if it were attempted to apply the construction shown in FIG. 2b to the Nb.sub.3 Sn fine multifilamentary superconducting wire, when the aluminum wires 11 and the Nb.sub.3 Sn fine multifilamentary superconducting wires 10 would be stranded together a large strain would be applied to the latter thereby seriously impairing the superconducting properties and thus making the composite unusable.
An example of a superconducting wire in which the Nb.sub.3 Sn superconductor is stabilized by aluminum is disclosed in an article entitled "ALUMINUM-STABILIZED Nb.sub.3 Sn MULTIFILAMENTARY WIRE FOR A HIGH-FIELD PULSE-MAGNET" by F. Irie et al., appearing in PROCEEDINGS OF THE INTERNATIONAL CRYOGENIC MATERIALS CONFERENCE, Kobe, Japan, May 11 to 14, 1982, pp 477-479. This superconducting wire, however, has such a cross-sectional construction that the core of Nb.sub.3 Sn filaments is surrounded by aluminum, so that it will be quite difficult to subject it to a desired drawing work, because the aluminum has an extremely higher plastic workability than the core.